[Valgrim]

Some Youtube folks tried a much simpler approach:

Take a large metal bucket or barrel.

Put a heating element and a simple oven thermostat on the bottom.

Fill with sand.

Connect to a solar panel or other enrgy source.

The air between the sand particles seem to actually provide a bit of convection and insulation.

The thermostat turns the circuit off before getting too hot for the heating element.

The heat accumulated during the day is slowly released over time directly through the metal.

[rkangel]

This is called a Storage heater (https://en.wikipedia.org/wiki/Storage_heater), usually referred to in the UK as a Night Storage Heater as it took advantage of cheap electricity at night to provide heat for a house.

Fitting Night Storage radiators was a much cheaper way of fitting central heating than the normal boiler and plumbing approach, and when electricity was cheaper it made more sense. Now, it makes no sense and if you buy a house with it the first thing you have to do is put in "proper" central heating. But maybe with excess renewables that will swing back.

[immibis]

Any system that involves an electrical solar panel connected to a low-temperature electrical heater is likely to be better served with a solar hot water panel, by about a 10:1 ratio.

[Retric]

More like 4.5:1 solar is 20% or so efficient and solar thermal is only like 90% when including losses.

Anyway, the real question here costs both in equipment and labor. Solar hot water panels involve plumbing and need radiators etc they quickly pay for themselves when heating a large home but don’t scale down very well.

Running the numbers I was surprised how cost competitive the sand bucket is for something like a chicken coop. Sure the panel(s) are wasted most of the year, but you don’t exactly need an electrician to set this up either. Probably also worth considering for redundancy in some situations.

[slow_typist]

The monetary case of PV does not account for the damage that is done to the environment over the entire lifecycle. Solar hot water panels are ecologically superior. Not only are they more efficient in W/sqm. Their production is not as energy intensive (plumbing included), the recycling process is less complex, poisonous materials can mostly be avoided. If heat is the desired product, they probably beat PV by an order of magnitude in the energetic dimension.

[Retric]

PV’s falling prices have also been associated with falling environmental harm. When you’re talking multiple orders of magnitude you just cannot require as much in production.

IE: You can’t burn 500 gallons of gas if the end product costs 500$. That applies not just to transportation but also how much material and thus mining the raw materials you need, including refining them, the amount of chemicals you can use per panel, how much electricity you can use in production of the device including precursors etc.

[slow_typist]

The argument is valid, if energy is not subsidised. However, burning pv for resistance heating is still wasteful and should be avoided. It is only acceptable for peak production that cannot be put to better use.

[Retric]

It’s wasteful of electricity not necessarily resources.

One of the numbers I was looking at compared air sourced heat pump at night when it’s coldest vs this kind of resistive heat battery. Solar panels are far cheaper and better for the environment on a kWh/day basis so even if the COP is 3 (or less it gets colder at night) * 90%(losses from battery) = fewer panels you more than offset it by needing far more batteries.

Obviously solar thermal setups have advantages if you need lots of heat, but they are also wasted most of the year. If 8-10 months a year you’re only using them for hot water then annual efficiency is closer to 25% than 90%.

[adrianN]

You can run a heat pump with the electricity. That changed the calculation a bit.

[slow_typist]

Sure but a heat pump was not the scenario in the parent post.

[mercutio2]

Are you including the maintenance costs of the water panel in your estimate?

I used to buy this argument, but then:

A) PV panels got ridiculously cheap

B) everyone I know with solar hot water has emptied their systems because the maintenance hassles were not worth dealing with

Using high grade intermittent current to produce resistive heat isn’t high on my list of efficient things to do, but unfortunately neither is maintaining hot water panels.

[defrost]

Might be a location thing; there are vast numbers of solar hot water systems here in W.Australia and typical maintainence is maybe just replace the tank outright and flush the lines every 20 years or so.

How 'clean' of salts, etc. is the water being put through your panels?

Hard water clogs up faster.

[mattclarkdotnet]

As you allude to, our water in WA is notably soft, maybe that helps? I mean it's full of iron but that mostly just seems to cause staining not clogging.

That said, a heat pump running at 4:1 COP coupled to 20% efficient solar panels gets you right back to the same efficiency as solar thermal, with a lot more flexibility.

[defrost]

I'd rather run both in parallel, and I do, as do most of the people here abouts.

No single point of failure, sun heats the water directly and provides power, with a breakout box that accepts power in from the grid (if required), exports excess for points, hopefully that gets better over time, and accepts a local generator input if the PV panels are offline for some reason when there's a local grid power outage.

This is pretty good for now, there's loose neighbourhood discussion about perhaps getting a local area battery in a sea container that can buffer ~200 standard homes to further secure the town's energy stability.

Flexibility, in rural settings, is about having options not a single point of failure | dependency.

Eg: Way up the hill it's good to have PV panels on the bore pumps and better to have these independant of the house circuits with cables in place to route power "in case" .. along with option to use a generator if needed.

[mattclarkdotnet]

Totally agree. I'm in the city so don't have roof space for both. If I could I would totally do it.

[stdbrouw]

Yeah, when I was looking into this a couple of years ago, thermal panels were about 4 times as efficient as photovoltaic panels, but they were also 4 times as expensive. The ratio has probably shifted in favor of photovoltaics since then. If you have very limited space on your roof solar thermal can still be a good idea, but otherwise why prefer low grade energy (heat) over high grade energy (electricity) if you can get the same capacity at the same price?

[palmfacehn]

The most I've had to do is clear the dust from the tubes during the dry season.

[KennyBlanken]

Solar hot water hasn't been viable for a decade or more; the ROI is piss poor. Efficiency falls as the water heats, and once your storage container is hot enough, the panels are useless.

At least in residential and commercial installations, you get a much higher ROI by putting in solar electric, using the electricity to power your home/facility, and dumping the excess into the grid to earn money/credit.

Resistive heat storage is also a thing these days; hundred-plus gallon tanks that will take power from the panels if it's more cost effective than returning it to the grid or the grid doesn't have the capacity to take it. That water then feeds a second water heater which brings it up to the final temperature, if necessary.

The efficiency relative to area doesn't really matter, as rooftop space is rarely at a premium.

[bb123]

But this would stop you getting the sand hotter than 100c - which is one of the main advantages of using sand over water for heat storage.

[kgabis]

You can make it work under pressure and/or add salt.

[omegabravo]

You would need a pressure vessel and corrosion resistance, and potentially a pump driving up the cost.

solar panel with sand is mechanically and electrically much simpler

[kiney]

Thats just a regular storage heater...

[prometheus76]

I'm toying with the idea of building one of these to use as a pool heater. Has anyone here tried this?

[onlyrealcuzzo]

Why not just do solar heating?

[prometheus76]

Because I have sunlight for a solar panel year-round, whereas I only have solar heat for a limited time.

[samstave]

Seems like according to everyone above - why not just use the water from your pool.

[Dalewyn]

For some reason this description sounds exactly like a computer, and specifically the CPU/GPU, and I love it.